As for First person shell casing, im assuming your talking new vegas, i may be wrong but in new vegas its hardcoded in fps, so you wont be able to change anything there i have no clue about sizes conversion to tps, but shell life length on the ground for tps is a easy variable change. if thats what you ment by that but im unable to help you with geck, i have no experience actually creating mods. Shell casing in first person is unchangable, you cant modify it in anyway, or any shells ejected while in fps will not stay longer than default hard coded time if viewed after wards in tps, I hope you understand. Third person shell casing is only changable/ moddable.